Heretofore, one type of PEM fuel cell power plant utilizes compressors to pump the reactant air to the cathode reactant gas flow fields, and has typically been operated with air inlet pressures of two or three atmospheres. This takes advantage of the ability of a fuel cell to operate at higher average cell temperatures, exhaust dew point temperatures and coolant temperatures as a consequence of higher operating pressures. Another type of known fuel cell power plant may utilize a fan or blower to provide the reactant air, and consequently works near ambient pressure. While this type of fuel cell power plant cannot take advantage of the efficiencies that result from higher pressure and therefore higher temperature, the overall system efficiency is higher than that of the fuel cell power plants operating at two or three atmospheres with a compressor, due to the extremely low parasitic power required of the blower or fan compared to the parasitic power required by compressors.
It is known that water removal from a fuel cell power plant is controlled by the temperature, pressure and volume of the process exhaust streams leaving the fuel cell power plant. The volume of the process exhaust streams are related to the reactant utilizations and the composition of the fuel used to operate the fuel cell power plant. Process exhaust is defined to be the sum of any exhaust streams from the fuel or oxidant sides of the fuel cell power plant. The fuel stream may be burned prior to exhausting the power plant as is known. The process exhaust from a hydrogen-fueled fuel cell is primarily unreacted air since the fuel utilization approaches 100% to maximize power plant efficiency. The process exhaust from a gasoline-fueled fuel cell includes significant quantities of carbon dioxide and nitrogen, which are by-products of the fuel reforming process. For a partial oxidation reformer, the exhaust volume from the fuel side of the system is approximately equal to exhaust volume from the air side of the system. The greater volume of the process exhaust stream in a gasoline-fueled fuel cell power plant vs. a hydrogen-fueled fuel cell power plant requires a lower system exhaust dew point for the gasoline-fueled fuel cell power plant to maintain water balance.
It is known that the thermodynamic efficiency of PEM fuel cell power plants increases as the fuel and oxidant inlet pressures increase. One of the benefits of higher operating pressure is that it increases the exhaust dew point at which water balance can be maintained, as is illustrated in FIG. 1. In FIG. 1, the leftmost five traces 13 illustrate the exhaust dew point temperature as a function of process exhaust pressure of a gasoline-fueled fuel cell power plant for pressures between one atmosphere and two atmospheres. The rightmost five traces 14 illustrate the increase in system exhaust dew point with increases in process exhaust pressure of a hydrogen-fueled fuel cell power plant for pressures between one atmosphere and two atmospheres. At water balance, water removed in the process exhaust stream is just sufficient to balance the water created at the cathode by the fuel cell process, and leaves adequate water to maintain sufficient humidification of the membrane. Providing process exhaust with the proper dew point, so as to achieve water balance, eliminates the need for additional components such as condensers and enthalpy recovery devices which add weight, volume and complexity to the overall system. When a system is operated essentially at the water balance exhaust dew point, water is nonetheless cooled and returned to the water inlet channels to ensure adequate presence of water thereby permitting water removal to achieve balance. Provision of a higher process exhaust temperature permits operation at a higher cell temperature and coolant temperature, and utilization of a smaller radiator for removal of heat from the coolant to ensure that the water returning to the water flow fields is at a lower temperature than the process exhaust (or operation within higher ambient temperatures), typically reducing overall system cost and weight.
It is known that the overall system efficiency must take into account efficiency of electrical and mechanical components exterior of the fuel cell power plant. In particular, utilizing electricity generated by the fuel cell power plant to operate the pump (fan, blower or compressor) which provides the process air to the fuel cell power plant comprises a significant load of parasitic power consumption. It has heretofore been known that a fuel cell power plant operating at or near ambient pressure utilizing a relatively low power pump has an overall system efficiency which is greater than fuel cell power plants operating at two or three atmospheres, which have higher thermodynamic efficiency, but lower overall system efficiency, due to the increased parasitic power requirement of the required air pump.